Hi "Young Sea Cat",
Reefer-boxes are refrigerated containers. They got their own refrigeration unit and the are powered, when on board, by plugging them into the ships electricity network (more or less the same as you would plug in your home's fridge into to you home network, but of course then different).
These reefers, as they are called, can contain all kind of perishable cargo such as fruit but also frozen stuff like meat, poultry etc. but then also chemicals that needs to be transported at a certain temperature.
On board we get so called manifests stating the kind of cargo, but you on land will not find out the contents unless you have contacts with shippers who offer their cargo to a container line for transport.
Regarding the Dangerous goods we on board also receive manifests stating the nature of the cargo with all kind of extra information in case of an emergency, i.e. fire and/or spill.
Cheers,
Roy